For well over 75 years the internal combustion engine has been mankind's primary source of motive power. It would be difficult to overstate its importance or the engineering effort expended in seeking its perfection. So mature and well understood is the art of internal combustion engine design that most so called "new" engine designs are merely designs made up of choices among a variety of known alternatives. For example, an improved output torque curve can easily be achieved by sacrificing engine fuel economy. Emissions abatement or improved reliability can also be achieved with an increase in cost. Still other objectives can be achieved such as increased power and reduced size and/or weight but normally at a sacrifice of both fuel efficiency and low cost.
An engine's fuel system is the component which often has the greatest impact on performance and cost. Accordingly, fuel systems for internal combustion engines have received a significant portion of the total engineering effort expended to date on the development of the internal combustion engine. For this reason, today's engine designer has an extraordinary array of choices and possible permutations of known fuel system concepts. Design effort typically involves extremely complex and subtle compromises among cost, size, reliability, performance, ease of manufacture and backward compatibility with existing engine designs.
The challenge to contemporary designers has been significantly increased by the need to respond to governmentally mandated emissions abatement standards while maintaining or improving fuel efficiency. In view of the mature nature of fuel system designs, it is extremely difficult to extract both improved engine performance and emissions abatement from further innovations in the fuel system art. Yet the need for such innovations has never been greater in view of the series of escalating emissions standards mandated for the future by the United States government. Meeting these standards, especially those for ignition compression engines, will require substantial innovations in fuel systems unless engine manufacturers are prepared to adopt significantly more costly fuel systems and/or engine redesigns. For example, Cummins Engine Company, Inc., assignee of the subject application, presently manufactures a pair of mid-range compression ignition engines identified as the B series and C series (5.9 and 8.3 liters displacement respectively). These engines employ a state of the art pump-line-nozzle (PLN) type of fuel system provided to Cummins by another manufacturer. However, this type of fuel system will not permit the B and C series engines to meet the future emissions abatement standards imposed by the United States government.
Among the universe of known fuel systems are several concepts which would appear initially to provide a possible solution to the requirement for improved emissions abatement and satisfactory engine performance. However, for the various reasons outlined below these systems are inadequate.
One possibility pioneered by the assignee of this invention is disclosed in U.S. Pat. No. 5,042,445 to Peters et al. This patent discloses a cam driven unit injector designed to provide very high injection pressures (30,000 psi or higher) even at low engine speeds. Such high injection pressures promote better fuel vaporization during injection thereby helping to assure complete combustion and thus reduced emissions in the engine exhaust. Implementation of this concept requires a unit injector (defined as a single unit device combining a fuel injection nozzle and high pressure pump) adjacent each engine cylinder wherein the injector is designed to achieve the desired high injection pressure at low engine speeds. The Peters et al injector is equipped with a hydraulic variable length chamber for controlling the timing of each injection event in response to engine conditions. Excessive pressures are avoided in this type of injector at elevated engine speeds by the provision of a pressure relief valve for dumping timing fluid during the injection stroke of the unit fuel injector.
Other types of unit fuel injectors are known which are capable of adequate high pressure injection and sufficiently precise injection to achieve some of the performance objectives discussed above. One example is disclosed in SAE Paper No. 911819 relating to a PDE unit injector developed by Bosch. Still another is disclosed in U.S. Pat. No. 4,531,672 to Smith assigned to the assignee of this application.
While the unit injectors described above are capable in many ways of achieving the desired performance objectives, major cost penalties are associated with adoption of such injectors on pre-existing engine designs. In particular, retro-fitting an existing engine such as the Cummins B series or C series engine with one of the above described unit injector designs would require a major overhaul of the engine. In particular, when these types of injectors were considered for the B and C engines, it became clear that a redesigned block, head, front end and all associated parts would be required. In short, a substantially new engine would be required with an attendant retooling investment in excess of several hundred million dollars.
Another approach for achieving the desired high pressure injection and variable timing required to meet the escalating emissions limitation standards is disclosed in a fuel system offered by Bosch under the designation PLD. This design approach is characterized by the provision of a separate high pressure pump unit associated with each engine cylinder and connected through a short line to a nozzle arranged to inject fuel into the associated cylinder. Each pump unit is individually packaged separate from the associated nozzle and from all other pump units associated with the engine. The pump units are mounted on the engine for actuation by the engine cam shaft as close as possible to the associated engine cylinder. Although this approach has fuel system cost and performance advantages resulting from the use of existing engine components and minimal impact on the head design, major changes would be required in the engine block. More particularly, the block would need to be entirely redesigned to accommodate the attachment of the individual pump units along the engine cam shaft. Implementation of this approach on the B and C engines would require an investment estimated to be in the neighborhood of several tens of millions of dollars.
One high performance approach requiring less engine redesign is disclosed in U.S. Pat. No. 5.096,121 to Grinsteiner. This style of unit injector includes a fluid pressure intensifying piston which has the effect of multiplying the pressure of a motive fluid, such as pressurized lubrication oil, by the ratio of the effective cross sectional areas of the intensification piston contacted on its larger, low pressure side by the motive fluid and on the smaller, high pressure side by the engine fuel. Such a design has the potential for achieving many of the desired performance objectives but some significant redesign of the base engine is still required. For example, the system requires an entirely new cylinder head to accommodate not only the injector but also the oil accumulator that provides the intensification. A separate lubrication circuit or a totally redesigned lubrication circuit must be provided to supply the motive fluid through a control valve to the intensification piston. Such an system would require a separate suction tube, oil pump, and filtration system.
The cost for base engine redesign required by a fluid intensification unit injector is likely to be considerably less than the engine redesign costs associated with adoption of any of the other unit injector and unit pump concepts described above. Nevertheless, Cummins estimates that adoption of fluid intensifiers on the B and C series engines would still require an investment in the range of multiple tens of millions of dollars. In addition to the costs associated with redesign of the engine, the fuel system itself including the hydraulic unit injectors, redesigned lubrication circuit, filters and associated equipment would likely be far more expensive than many other known types of fuel systems. U.S. Pat. Reissue No. 33,270 to Beck et al. discloses another type of hydraulic intensifier unit injector which would appear to supply the same benefits but suffer the same drawbacks discussed above.
Yet another approach to meeting the goal of increased fuel system performance would be to provide an accumulator for storing the output of a high pressure pump and to provide a plurality of injection nozzles connected with the accumulator and associated with the engine cylinders wherein each nozzle includes a separate integrated solenoid valve to control the timing and quantity of fuel flow from the accumulator into each cylinder. Examples of this type of system are disclosed in U. S. Pat. No. 5,094,216 to Miyaki et al. and SAE article no. 910252 entitled Development of New Electronically Controlled Fuel Injection System ECD-U2for Diesel Engines by Miyaki et al. This system allows the accumulator pressure (and thus the injection pressure) to be regulated as necessary independent of engine speed. However, solenoids capable of handling the very high pressure and the necessary fast response times are relatively bulky and costly. Such solenoids will require severe head redesign on the C series and some modification on the B-series engines. Also, mounting of the high pressure accumulator on an internal combustion engine is not necessarily simple nor does it yield an uncluttered engine package or appearance. While the total engine redesign costs would be less than the engine redesign costs associated with adoption of the fuel systems noted above, the costs associated with the fuel system components themselves, including the high pressure pump and solenoid controlled injection nozzles, could be prohibitively high.
The above described approaches could potentially achieve many of the desired performance objectives but a major cost penalty is associated with each design either in the form of a costly engine redesign or added fuel system costs or both. Other less costly fuel system concepts are known but these concepts fail to provide the full complement of performance objectives desired.
One approach which would require virtually no engine redesign involves the provision of a high pressure "in-line" pump such as offered by Bosch under the designation P7100. In this type of system injection nozzles located at each engine cylinder are connected through separate lines to corresponding pumping chambers contained within the housing of a single unitized high pressure pump. The chambers are aligned along the axis of a pump drive shaft and contain corresponding plungers mounted to be reciprocated by the pump drive shaft in synchronism with the engine crankshaft. With appropriate design and controls, in-line systems of this type can achieve the necessary pressures and injection accuracy under some engine conditions but can not be relied upon to provide the desired performance objectives over the long term especially at low engine speeds. Further, in-line fuel pumps which are capable of approaching some of the more important pressure and control objectives are enormously more expensive than the present pump line nozzle system used on the Cummins B and C series engines.
Another fuel system which would necessitate little redesign of the basic engine involves the use of a rotary pump design. This type of pump is characterized by a pump housing containing a plurality of radially oriented pump chambers within which are mounted plungers adapted to be reciprocated by a cam surface located at the center of the pump housing. U.S. Pat. Nos. 4,498,442 and 4,798,189 disclose examples of this type of pump. Although engine impact is low and cost is relatively low, rotary pumps lack performance capability at higher engine ratings. In particular, rotary pumps are not capable of providing the desired volume or the desired high pressure over the full operating range of a typical engine.
Still another fuel system concept is disclosed in Japanese Pat. Application Document 57-68532 to Nakao and assigned to Komatsu. This reference discloses an electronically controlled high pressure pump and an accumulator for receiving the pump output for supply of a plurality of injection nozzles through a distributor type valve and corresponding fuel supply lines. The timing and quantity of injection is controlled by means of rotary valve elements combined with the distributor valve. The pressure within the accumulator is regulated by a feedback signal responsive to the accumulator pressure to control the effective displacement of the high pressure pump. While this design has features of interest, it fails to disclose how to achieve the necessary operating pressures in a unitized assembly of sufficiently compact size to allow the resulting system to be mounted in a practical manner on an internal combustion engine. No provision is made for operating the system in a fail safe manner in case one or more of the electronic control mechanisms should fail during operation. Furthermore, the design provides for an entirely separate pump assembly and accumulator components connected by a plurality of separate fluid lines which would multiply the sites of potential leaks.
The Komatsu reference also fails to teach how to manufacture in a practical manner an accumulator so that the very high pressures, i.e. 5,000 to 30,000 psi or higher, could be stored within a compact package having adequate fuel storage capacity with freedom from potential leakage or dangerous failure. The Komatsu reference further fails to suggest how to design and assemble the system to achieve an acceptably low manufacturing cost. The disclosed distributor valve would also not be suitable for handling the very high pressures required for the system without simultaneously giving rise to a high probability of fuel leakage that would cause excessive parasitic loses, that is an excessive amount of mechanical energy would be required to drive the fuel system pump that would otherwise be available as useful output from the engine.
Still other references have disclosed the concept of providing an accumulator in a fuel system wherein fuel from the accumulator can alternatively be controlled for injection into the respective engine cylinders either by a distributor valve or a plurality of solenoids associated with each of the individual injector nozzles. German Printed Pat. Application No. DE 3618447 Al assigned to Bosch discloses an example of this type of system. The highly schematic disclosure of this teaching, however, causes this reference to fail to teach how to solve the problems referred to with respect to the Komatsu reference.
Attempts have been made to design a high pressure common rail or accumulator for storing the output of a high pressure pump for delivery to injection nozzles. For example, U.S. Pat. No. 5,109,822 to Martin discloses a high pressure common rail fuel injection system including a common rail formed from a one-piece metal housing having a series of elongated bores formed therein for temporarily storing the high pressure fuel delivered by a high pressure pump. However, Martin fails to teach how to determine the optimum arrangement of elongated chambers or bores for producing a compact common rail with minimum outer dimensions which fit within existing available mounting envelopes required by existing engines while ensuring that the common rail housing walls are sufficiently strong to withstand the forces generated by the very high operating pressure of the fuel in the chambers. In addition, Martin does not disclose how to ascertain the minimum required fuel storage volume for the common rail which is a primary factor in designing a compact common rail. Also, the common rail disclosed in Martin is not integrated with the high pressure pump unit and/or other components, such as a fuel pump control valve, to form a compact fuel delivery assembly which is capable of efficiently controlling the pressure in the common rail. U.S. Pat. No. 2,446,497 to Thomas discloses a high pressure pump, a common high pressure chamber or accumulator, a distributor and fuel injection control governors mounted adjacent one another to form a combined fuel injection assembly. However, Thomas fails to disclose a fuel assembly which is highly compact and integrated, and also capable of efficiently and effectively controlling both the pressure in the accumulator and injection timing and quantity.
Attempts have also been made to design high pressure, high speed solenoid operated valves for use in fuel systems for compression ignition internal combustion engines. For example, U. S. Pat. No. 3,680,782 to Monpetit et al discloses an electronically controlled fuel injector employing a force balanced three-way valve having a nearly force balanced "pin-in-sleeve" valve member design. In valves of this type, the movable valve member is movable between first and second positions to alternatively connect an output valve passage to one of two alternative valve passages, typically a high pressure source and a drain. The movable valve member contains a cavity opening at one end to telescopingly receive a floating pin. A first valve seat is formed between the sleeve and the surrounding valve housing and a second valve seat is formed between the sleeve and pin. The valve element is movable between a first position in which the injector nozzle is connected with a source of fuel under high injection pressure and a second position in which the valve element isolates the source of fuel from the injection orifices of the nozzle and connects the passage leading to the injection orifices to a drain to insure near instantaneous termination of each injection event.
Other examples of three-way high speed, high pressure fuel system valves are disclosed in U.S. Pat. No. 5,038,826 to Kabai et al (Nippondenso). While capable of handling high pressure and operating at high speed, the "pin-in-sleeve" arrangements of the Monpetit et al. and Nippondenso references do not permit the effective valve seats of each disclosed design to be substantially unequal in size while maintaining the valve member substantially force balanced.
Another important feature of an effective fuel delivery system is the ability to regulate the injection pressure as necessary independent of engine speed. U.S. Pat. No. 5,094,216 to Miyaki et al. and U.S. Pat. No. 4,502,445 to Roca-Nierga et al. both disclose a plural chamber "in-line" fuel pump assembly having an output control device which varies the effective displacement of one or more pump plungers by providing a separate pump control valve for each pump chamber which operates to vary the beginning of injection with a constant end of injection occurring when the pumping plunger reaches its top dead center position. Specifically, fuel is supplied to the pumping chamber during the retraction stroke and then pumped out of the pumping chamber during the advancing or pumping stroke until the control valve is closed blocking the discharge of fuel from the chamber thereby commencing injection or delivery. The delivery or discharge from the pumping chamber is finished only at the end of the pumping stroke of the plunger.
Yet another important feature of an effective fuel delivery system capable of meeting the ever increasing requirements of emissions abatement is the ability to control the rate of fuel delivery during each injection event. It has been shown that the level of emissions generated by the diesel fuel combustion process can be reduced by decreasing the volume of fuel injected during the initial stage of the injection event. One method of reducing the initial volume of fuel injected during each injection event is to reduce the pressure of the fuel delivered to the nozzle assemblies during the initial stage of injection. Various devices have been developed to control or shape the rate of fuel delivery during the initial phase of fuel injection so as to reduce the fuel pressure delivered to the nozzle assemblies. For example, U.S. Pat. Nos. 3,718,283, 3,747,857, 4,811,715 and 5,029,568 disclose devices associated with each injector nozzle assembly for creating an initial period of restricted fuel flow and a subsequent period of substantially unrestricted fuel flow through the nozzle orifice into the combustion chamber. However, these rate control devices require modifications to each of the fuel injector assemblies in a multi-injector system thus adding costs and complexity to the injection system. U.S. Pat. No. 4,469,068 to Kuroyanagi et al. discloses a distributor-type fuel injection apparatus including an variable volume accumulator to vary the rate of fuel injection to achieve effective combustion. However, this device uses a complex accumulator control system to vary the rate of injection which is specifically designed to be used with a distributor having a reciprocating plunger.
Distributor-type fuel injection systems are also subject to another undesirable phenomena known as secondary injection. When the nozzle element of the nozzle assembly closes at the end of each injection event, reverse pressure waves or pulses are generated which travel back upstream in the fuel delivery lines to the distributor or delivery valves. Under certain operating conditions, these pressure waves may be reflected back toward the nozzle assembly by the distributor or delivery valve creating a secondary nozzle operating pulse of sufficient magnitude to cause the nozzle valve to lift from its seat causing an undesired secondary injection. U.S. Pat. No. 4,246,876 to Bouwkamp et al. discloses a conventional "snubber valve" used to dampen or diffuse the pressure wave energy traveling from the nozzle valve thereby preventing secondary injection by minimizing the intensity of any resultant reflected pressure wave. However, this design requires a separate snubber valve to be used in each fuel injection line thus adding cost to the system. U.S. Pat. Nos. 4,336,781, 4,624,231 and 5,012,785 all disclose rotary distributor fuel delivery systems using a single snubber-type valve positioned in the rotary shaft of the distributor to dampen pressure waves in each injection line.
In order to achieve accurate and predictable injection quantities of fuel during each injection event, it is important to ensure that the fuel transfer circuit connecting the fuel supply to the nozzle assemblies is continuously full of fuel. It has been found that vapor pockets or voids (called cavitation) in the transfer circuit result in insufficient injection pressure and variations in both fuel quantity and timing of injection. Vapor pockets or voids are especially prone to be formed in high pressure lines of fuel systems where such lines are connected to a low pressure drain. When the fuel transfer circuit, and thus an injection line, is connected to drain at the end of the injection event, fuel at one end of the injection line exits out of the nozzle while fuel at the other end of the circuit exits to drain thus rapidly drawing fuel away from, and reducing the pressure in, intermediate portions of the circuit and injection line. This effect can result in the formation of a vapor pocket or void in the fuel transfer circuit and injection line between the drain and nozzle. Snubber valves, mentioned hereinabove with respect to the prevention of secondary injections, are also used to prevent excessive cavitation by allowing substantially full flow through an injection line to an injector while restricting the return flow of fuel from the injector thereby maintaining fuel in the fuel delivery lines. For example, Japanese Pat. Publication 05-180117 discloses a damping valve positioned downstream of a delivery valve for preventing cavitation erosion. The damping valve includes a spring-biased valve element having an orifice and a pressure regulation valve positioned in a bypass channel. This device appears to regulate the fuel pressure in the fuel injection line between the damping valve and a fuel injection valve to below a preset maximum.
In short, the prior art does not provide a practical, low cost fuel system which satisfies the conflicting demands of emissions control and improved engine performance especially in situations where it is desired to retrofit a pre-existing engine design. Moreover, there does not exist those fuel system components (such as accumulators, solenoid valves, and injection control valves) having all the characteristics necessary for providing fuel under extremely high pressure in precise quantities at precise times as determined by controls that are responsive to a wide range of engine conditions.